Over 2.2 million Americans, or 8% of those 65 years and older, have difficulty with ambulation. The ability to ambulate safely on even and uneven surfaces, to successfully negotiate steps, obstacles and restricted spaces is an important aspect of independent living. A major focus of this research is understanding how the biomechanical requirements of ambulation tasks match the individual's biomechanical capabilities as changes to both occur with increasing functional impairment. Thus the biomechanical requirements of the seven (Project 1 Lo-tech) ambulatory tasks will be quantified in 126 (Project 1) subjects with a range of impairments. These data will then be used in Program-wide tests of hypotheses concerning the role of biomechanical and neuropsychological factors in mobility impairments. In 100 other elderly subjects (aged 55-69 years) with and without Parkinson's Disease, who also have a range of mobility impairments, detailed "Hi-tech" biomechanical analyses will be used to study how ambulatory task performance depends upon the interaction between biomechanical requirements and physical/ cognitive impairment(s). Three ambulatory activities will be used: a highly-modified walking-in-place task, stepping over obstacles, and walking up and down stairs. The first task is designed to give insights into how, when and why the threshold tolerance for whole body instability, hypothesized to be a fundamental requirement of normal gait, changes with impairment. The second task is designed to investigate why tripping is a leading cause of falls in the elderly: the hypothesis will be tested that impairments in peripheral vision, biomechanical and/or neuropsychological factors such as attention can adversely affect obstacle clearance success rates, hence increasing the probability of tripping. The third task is designed to determine whether it is the increased strength requirement alone, or an interaction with physical or neuropsychological deficits, that causes elderly subjects capable of ambulating on the level to have stair mobility problems. We will also analyze how the use of a right hand-rail alters task requirements and performance. The insights gained from this research will improve the effectiveness of diagnostic, therapeutic and prophylactic measures in maintaining ambulatory independence of the elderly as long as possible. Because of their rich detail, and graded difficulty structure, these analyses will yield significant new insights into the biomechanical elements of mobility.